Borderline Personality Disorder (BPD) is a serious mental illness that affects individuals, families, and communities and has enormous public health costs. Psychotherapy continues to be the necessary and primary treatment modality for BPD and the development of efficacious and effective interventions is therefore of critical importance. The overall objective of the proposed research is the pilot testing of a theoretically-based 12-week group intervention that is finely tailored to the specific needs and problems of individuals diagnosed with BPD. Specifically, based on a model of BPD as a disorder of emotional deficits (emotion dysregulation and experiential avoidance) that occur primarily in interpersonal contexts, this novel treatment program utilizes interpersonally-focused acceptance and mindfulness-based strategies to actively target these deficits and build adaptive emotional and behavioral skills. My central hypothesis is that, by targeting emotion dysregulation and experiential avoidance, the proposed treatment will lead to significantly greater reductions in BPD symptoms than an attention placebo control group. There are 2 primary aims and 1 secondary aim to the proposed research, all to be examined in a clinical sample of individuals diagnosed with BPD: Aim 1 is to test the ability of the proposed treatment, compared to an attention placebo control, to impact BPD symptoms. Aim 2 is to test the ability impact of the proposed treatment, compared to an attention placebo control, to impact the targeted process variables (emotion regulation and experiential avoidance). Aim 3 is a secondary aim to examine the impact of changes in the targeted process variables (emotion regulation and experiential avoidance) on changes in BPD symptoms. Data on proposed outcome and process variables will be measured at baseline, as well as at regular intervals during the 12-week treatment and the 6-month follow-up period. Utilizing multilevel growth curve statistical methods, this approach will allow me to identify if and when: (a) a sufficient dose of therapy has been given to change BPD symptoms, (b) changes in the process variables occur due to treatment, (c) changes in the process variables precede changes in BPD symptoms, and (d) positive changes are maintained after treatment. This application represents the first step toward achieving the long-term goal of disseminating efficacious, accessible, and feasible treatments for BPD, based on empirically-supported theoretical models and active treatment components. If the effectiveness of this treatment is confirmed, future studies will include larger samples to help test whether specific subgroups will preferentially respond, and comparisons to other programs, including Dialectical Behavior Therapy.